Physical NIC linkstate is down.
Failure of the network cable, disconnect of cable, failure of the physical network card, failure of network switch, port, and so on.
Check the physical connection of the vmnic specified in the alert, and the configuration of the attached network switch.
Note that by browsing the Veeam view _vCenter Storage Network Topology you can see which virtual switch is impacted, and which VMs are attached to the impacted switch.
Refer to this VMware KB article for more information on troubleshooting network connectivity issues.
Refer to the VMware Online Documentation for more information.